Thanks to their superior mechanical properties such as hardness, low coefficient of friction, thermal stability at high temperature, oxidation resistance, inertness to most common chemicals, and electrical and optical properties, ceramic materials are technologically important materials for many applications, especially for high temperature, high power, electronic barrier coatings, and cutting tool applications. For example, tungsten carbide (WC) including WC-Co alloys, silicon nitride, aluminum oxide, silicon oxide, titanium nitride, and titanium carbide are all very popular materials in the market, particularly in machining business.
As used herein, a xe2x80x9ccerarnics materialxe2x80x9d includes nitrides, oxides, borides, sulfides, carbides, suicides, carbon, hydrogenated carbon materials and the like.
For example, titanium nitride has been exceptionally popular among other ceramic materials. Due to its beautiful golden color, it has been widely used for decoration and marketing purposes. It is also very hardxe2x80x941770 kg/mm2, stable in air at temperature as high as 650-degC. It is relatively corrosion- and wear-resistant with relatively low friction coefficient, making it very popular in cutting tool industry. Various high temperature PVD and CVD titanium nitride coated cutting/machine tools (such as inserts, drills, saws, punches, dies, molds, pins, knifes), fuel injector components are currently available in the commercial markets. In fact, most of the above components are made of high speed tool steel. The material itself is expensive relative to lower grade steel materials. Taken diesel fuel injector components as an example, the diesel fuel injection operation is carried out at very high pressure, normally at about 150,000 psi (pounds per square inch) for diesel fuel engine. The injection is also carried out at very high speed and in very short cycle time. The excessive failures are resulted due to fatigue from repeated impact, extensive wear and erosion caused by abrasive particles. In order to prolong the life of the fuel injectors, high speed tool steel is commonly used, with extensive post-treatments, including heat treating which increases hardness; tempering to relieve stresses and restore toughness; surface nitriding; and finally coating with titanium nitride. The entire post treatment costs approximately three times as much as the combined cost for material and fabrication process (including finish grounding). The titanium nitride coating alone costs almost half of the total components. The key for cost-reduction is to lower post-treatment cost, especially coating cost.
There are basically two major kinds of nitriding methods: gas phase nitriding and liquid bath nitriding. In the gas phase nitriding process, nitrogen-containing gases, mostly ammonia or nitrogen are used. Under high temperature or high energy radiation, e.g. DC electric, RF radiation, ammonia or nitrogen are dissociated into nitrogen atoms. The highly energetic and active nitrogen atoms reacts with metal surface and diffused into the subsurface to form metal-nitrides.
If hydrocarbon is also introduced along with nitrogen and/or ammonia, the process is then called carbonitriding or nitrocarburizing. In the like manner, surface carburizing is performed with hydrocarbon containing gases.
The other method uses liquid bathxe2x80x94actually a molten nitrogen-containing salt bath. The molten salt reacts with the metal surface to form nitrides, carbides, and oxides.
These nitriding, carburizing and carbonitriding processes may form up to millimeter deep nitride, carbide material over extended period of time.
In general, heat treatment refers to a defined process which heats and cools a solid metal or alloy in a controlled manner in order to improve specific properties, such as hardness, strength, ductility, magnetic susceptibility, toughness, machinability, fabricability, and even corrosion resistance. According to this definition, heat treating has long been practiced since ancient metalsmith or blacksmith made bronze tools and iron-based swords. Metals are still heat treated today, for the same reasons as ancient daysxe2x80x94namely, to enhance or maximize mechanical properties of the materials. Optimizing hardness, strength, and other mechanical characteristics by heat treating techniques remains as primary interest of many equipment designers. Depends on composition and nature of the materials to be treated and their applications, heat treating may include austenitizing, quench hardening, annealing, tempering, normalizing, austempering, stress reliving, precipitation hardening, sintering, as well as surface enhancement techniques which utilizes thermochemical treatment such as carburizing, carbonitriding, nitriding, nitrocarburizing, etc.
For applications involving extensive wear, the most common practices today are to chose high grade (relatively expensive) steel as starting material, for example, high speed tool steel which offers better mechanical properties such as a good combination of hardness and toughness. After the components are machined to desired size, shape and surface finish, heat treatment is carried out to harden and temper the parts. In most cases, quench hardening may increase hardness of the treated part significantly, however, it makes the part brittle. Tempering is therefore necessary to increase toughness of the material, however, it sacrifices the hardness. It is difficult to balance between toughness and hardness because a part treated for maximum toughness is often too soft, whereas the same part treated for maximum hardness becomes too brittle. Surface nitriding is therefore employed to further improve the hardness and wear-resistance. Due to the increasing safety and environmental concerns, gas nitriding and plasma nitriding have been widely used today. In the plasma nitriding process, nitrogen-containing gas (nitrogen, ammonia) is dissociated and ionized. The nitrogen ions bombard metal surface, react and diffuse into the subsurface to form nitrided layer. The high energy plasma used in plasma nitriding (or ionitriding) also reduces substrate temperature.
After nitriding, a thin film of titanium nitride is often further applied to increase the hardness of the components. Since current titanium nitride coating techniques require high temperature, all problems associated with this technique, as described in the previous sections, become inevitable.
For certain applications such as machining, ceramic materials are often employed by utilizing the hardness of ceramic materials. For example, tungsten carbide based cutting inserts, drills, dies, and so on are gaining popularity. However, ceramic materials are very difficult to machine. Materials such as tungsten carbide often lack good tribological quality. Therefore, a layer of tribological coating such as TiN or composites would improve wear related performance.
These and other limitations of the prior art methods are addressed by the techniques of the present invention, which do not depend on a high grade material, which do not require fill range of heat treatment, which do not require high temperature and which do not adversely affect mechanical properties of the base materials.
The most popular process for titanium nitride coating is PVD method, as described in previous section. Its major limitations include (1) high substrate temperature which anneals substrate and causes dimension change and distortion; (2) complicated equipment and control operation; (3) extended preheating and cooling period; (4) long cycle time; and (5) coating-substrate adhesion is not as good as the coating produced with high temperature CVD method.
Chemical vapor deposition (CVD) has also been developed, though it is not as popular as PVD. A comprehensive review may be found in (F.S. Galasso, Chemical Vapor Deposited Materials, CRC press, FL, (1991)).
Although Musher and Gordon (xe2x80x9cAtmospheric pressure chemical vapor deposition of TiN from tetrakis(dimethylamido) titanium and ammoniaxe2x80x9d, J. N. Musher and R. G. Gordon, J. Mater. Res., 11 (4), 989-1001, (1996).) have recently demonstrated a low temperature CVD process to produce titanium nitride films on glass and silicon substrates at low temperature (190-420-degC.), the growth rate was only 25 xc3x85/min (25 Angstroms per minute) at 190-degC. (degrees Celsius) to 150 xc3x85/min at 420-degC., much too lower for mechanical applications, not to mention that the nitrogen/titanium ratios in the deposited films were significantly substoichiometric (N/Ti less than 0.6-0.75). In a paper titled xe2x80x9cThe influence of ammonia on rapid-thermal low-pressure metalorganic chemical vapor deposited TiNx films from tetrakis (dimethylamido) titanium precursor onto InPxe2x80x9d, (A. Katz, et al. J. Appl. Phys. 71 (2), 993-1000 (1992).) Katz et al. demonstrated a CVD process with improved growth rate of 1000 xc3x85/min on InP substrate at 300-degC.-350-degC. temperature range, however, (A. Weber, et al. J. Electrochem. Soc. 141 (3), 849 (1994).) Webe et al. also found that high growth rates led to poor quality films. Moreover, all above CVD processes were directed to semiconductor applications on Si or InP semiconductor substrates, coating-substrate adhesion was never a part of their concerns.
In summary, low temperature CVD titanium nitride coating processes have been demonstrated on semiconductor substrates. However, the growth rates are generally low, quality and adhesion are too poor to be practical for wear-related applications.
Moreover, no low temperature CVD titanium nitride process has been demonstrated on metal, alloy or ceramic substrates, such as steels, tungsten carbide based ceramics, etc.
Development of low temperature PVD processes has been attempted, one of the major problems is that adhesion between the coating and the substrate is too weak for wear-related applications
Presently, the TiN coating is mostly based on Physical Vapor Deposition (PVD) techniques. It introduces titanium atoms and ions from a solid titanium target surface using sputtering techniques. The titanium atoms and ions subsequently react with nitrogen-containing plasma. TiN is then deposited onto substrates, e.g. fuel injector components, which are in contact with the plasma. A more detailed process procedures include:
(1) load substrates into vacuum reactor;
(2) evacuate the reactor, using at least two pump stages (one foreline pump and one diffusion pump), to a base pressure of about 10-6 Torr;
(3) preheat the substrate to a desired temperature, normally 1100 F.;
(4) clean the substrate surface with plasma (normally hydrogen plasmas);
(5) initiate the nitrogen-containing plasma to start sputtering and deposition; and
(6) after deposition, the coated components need to be sufficiently cooled before venting the reactor to unload the components.
Current PVD titanium nitride coating processes require complicated and expensive equipment. The process also demands high precision and many parameters need to be strictly controlled. The substrate needs to be very clean, free of oil and water, otherwise, it would be difficult to pump to the ultra-high vacuum required. The high substrate temperature (about 1100 F.) required (to ensure a good bonding between the TiN coating and the steel substrate) is detrimental to the hardened steel substrates. It actually softens the already hardened steel and may change substrate dimensions at least in certain extent over the long deposition period. The deposition cycle is very long, normally around six hours for a 1-2 micron thick coating. It is not capable for thick coating, due to stress, it cannot make thick coatings such as 20 microns. It has very narrow parameter space, little pressure deviation would result in different phases of coating material. The high coating temperature makes it take long time to cool from 500-degC. to 200-degC. though you may cool it faster from 200-degC. to room temperature. The process also produces and exhausts safety and environmental hazards such as hydrogen, ammonia, etc. In addition, PVD is more difficult to control quality; requires more complicated equipment and control operation; extended preheating and cooling period hence long cycle time; and coating-substrate adhesion is not as good as the coating produced with high temperature CVD method in general.
Although it is not as popular as PVD, high temperature Chemical Vapor Deposition (CVD) has long been developed. As a matter of fact, CVD titanium nitride coating was developed earlier than its peer PVD process. However, CVD titanium nitride coating process was basically replaced by the corresponding PVD process because its following shortcomings:
(1) The CVD processes used TiCl4 as titanium precursor gas. Since titanium-chlorine bonds are very strong, high deposition temperature is required to dissociate the TiCl4 to form titanium nitride coating.
(2) Chlorine is always found in the film, deteriorating film quality.
(3) Chlorine and hydrochloric formed cause corrosion to the system and hazards to the environment.
Recently, low temperature CVD titanium nitride coating processes have been demonstrated on semiconductor substrates. Because they were intended for semiconductor application, the growth rates are generally low, quality and adhesion are too poor to be practical for wear-related mechanical applications. Moreover, no low temperature CVD titanium nitride process has been demonstrated on metal or alloy substrates, let alone steel.
TiN coating has also been demonstrated with low temperature PVD process, however, lower temperature sacrifices adhesion between the coating and the substrate, film quality and deposition rate.
Based on above analyses, it is evident that the key for cost-reduction is to lower post-treatment cost, especially TiN coating cost. A low temperature, high growth rate process would reduce cycle time, preserve toughness and other properties of core substrate materials, therefore would cut cost. Meanwhile, the enhanced adhesion and smoother surface would improve performance of fuel injector components. Therefore basic objectives of the invention may be summarized as follows:
1. Producing TiN coating at low temperature ( less than 350-degC.);
2. Smoother surface hence lower friction coefficient;
3. Enhanced adhesion;
4. Faster growth rate (6 micron/hour).
In addition, the high speed tool steel material itself is expensive relative to lower grade steel materials. Substitution of the tool steel material with lower grade, cheaper materials will further reduce cost.
Limitations of the prior art methods, as described in previous sections, are addressed by the present invention. Firstly, it does not require high temperature as other processes, therefore it does not adversely affect mechanical properties of the base materials. Secondly, smoother, more uniform coating produced by the present invention provides lower friction, enhanced adhesion characteristics, resulting in better performance. Thirdly, cycle time is reduced significantly due to the faster growth, elimination of loading, evacuation, substrate preheating and cleaning, as well as shorter cooling time. Additionally, the present inventive process is not sensitive to process parameters such as flow rates, pressure, temperature, etc. making it more controllable, reliable and reproducible. Another advantage is that potentially, more economic substrate material may be used to replace the current high speed steel, further cutting material cost. All these lead to lower production cost and better reliability. Finally, the present invention combines titanium nitride coating process with the existing plasma nitriding process, therefore, no additional equipment investment is necessary.
Metal-releasing agent: A metal-releasing agent is a metal-containing compound or complex in which chemical bonds around the metal atom(s) or group(s) are very weak and can be easily broken or dissociated so that metal atoms or desired metal-containing groups are released easily. Most of metal-releasing agents may be found among metal hydrides and organometallic compounds or complexes.
Nitrogen-releasing agents: A nitrogen-releasing agent is a nitrogen-containing compound or complex in which chemical bonds around the nitrogen atom(s) are very weak and can be easily broken or dissociated to form active nitrogen-containing species. Some materials may release both titanium and nitrogen simultaneously (as a leaving group).
Plasma assisting gas: A plasma assisting gas is a gas which assists initiation, stabilization of the plasma. It also assists activation (including dissociation and ionization) of precursor materials. Plasma assisting gases are usually inert gases in the group VIIIA consisting of helium, neon, argon, krypton, xenon.
Reactive gas: A reactive gas is a gas that participates chemical reactions during deposition processes. For example, in titanium nitride deposition processes, nitrogen, hydrogen or ammonia may be used as reactive gases.
Substrate: A substrate is a component, a part or a device to be coated. For example, fuel injector components in this proposal are considered as substrates.
Titanium-releasing agents: A titanium-releasing agent is a titanium-containing compound or complex in which chemical bonds around the titanium atom(s) are very weak and can be easily broken or dissociated to form active titanium-containing species. Most of titanium-releasing agents may be found among metal hydrides and organometallic compounds or complexes.
Prior Art References
Attention is directed to the following U.S. Pat. Nos. 5,288,543; 5,707,705; 5,399,379; 5,165,804; 5,312,529; 5,523,124; and 5,135,775.
Attention is directed to the following publications, discussed hereinabove:
1. Raaijmakers, et al. in Advanced Metallization and Processing for Semiconductor Devices and Circuits II, edited by A. Katz, et al. (Mater. Res. Soc. Symp. Proc. 260, Pittsburgh, Pa., 1993), p.99.
2. Tsai, et al. in Advanced Metallization and Processing for Semiconductor Devices and Circuits II, edited by A. Katz, et al. (Mater. Res. Soc. Symp. Proc. 260, Pittsburgh, Pa., 1993), p.793.
3. Chiang, et al. in Advanced Metallization and Processing for Semiconductor Devices and Circuits II, edited by A. Katz, et al. (Mater. Res. Soc. Symp. Proc. 260, Pittsburgh, Pa., 1993), p.813.
4. xe2x80x9cAtmospheric pressure chemical vapor deposition of TiN from tetrakis(dimethylamido) titanium and ammoniaxe2x80x9d, J. N. Musher and R. G. Gordon, J. Mater. Res., 11 (4), 989-1001, (1996).
5. xe2x80x9cThe influence of ammonia on rapid-thermal low-pressure metalorganic chemical vapor deposited TiNx films from tetrakis (dimethyamido) titanium precursor onto InPxe2x80x9d, Katz, et al. J. Appl. Phys. 71 (2), 993-10000 (1992).
6. Weber, et al. J. Electrochem. Soc. 141 (3), 849 (1994).
7. F. S. Galasso, Chemical Vapor Deposited Materials, CRC press, FL, (1991).
Based on above analyses, it is evident that industries have immediate needs to develop a process, material or coating to make metal or alloy surface hard, tough, wear-resistant. The processes are desired to operate at low temperatures ( less than 350-degC.) so that bulk properties are not adversely affected.
It is therefore an object of the present invention to provide an improved technique for fabricating a hard, wear-resistant ceramic coating on metal, alloy and ceramic surfaces without affecting bulk metal properties, especially core toughness, adversely.
It is another object of the present invention to provide a surface coating on metals, alloys and ceramics with improved toughness.
It is another object of the present invention to provide a surface coating that reduces friction.
It is another object of the present invention to provide a surface coating on metals, alloys and ceramics with enhanced corrosion and erosion characteristics.
It is another object of the present invention to provide a technique for high growth rate ( greater than 1 micron/hour, preferably about 10 micron/hour) ceramic film deposition and strong coating-substrate bonding.
It is another object of the present invention to provide a low temperature technique for fabricating nitride ceramics at low temperature (preferably under 250-degC.) and high growth rate ( greater than 1 micron/hour, preferably about 10 micron/hour) by using metal-releasing agents as precursors. The nitrides are preferably titanium nitride, silicon nitride, boron nitrides, aluminum nitride, carbon nitride, hafnium nitride, tantalum nitride, zirconium nitride, and other metal nitrides, etc.
It is another object of the present invention to provide a technique for fabricating oxide ceramics with CVD method at low temperature (preferably under 250-degC.) and high growth rate ( greater than 1 micron/hour, preferably about 10 micron/hour) by using metal-releasing agents as precursors. The oxides are preferably aluminum oxide, silicon monoxide, silicon dioxide, titanium oxide, zirconium oxide, tungsten oxide, molybdenum oxide, and other metal oxides, etc.
It is another object of the present invention to provide a technique for fabricating carbide ceramics with CVD method at low temperature (preferably under 250-degC.) and high growth rate ( greater than 1 micron/hour, preferably about 10 micron/hour) by using metal-releasing agents as precursors. The carbides are preferably aluminum carbide, silicon carbide, titanium carbide, zirconium carbide, boron carbide, hafnium carbide, molybdenum carbide, tungsten carbide, tantalum carbide, and other metal carbides, etc.
It is another object of the present invention to provide a technique for fabricating boride ceramics with CVD method at low temperature (preferably under 250-degC.) and high growth rate ( greater than 1 micron/hour, preferably about 10 micron/hour) by using organic boranes and metal-releasing agent as precursors. The borides are preferably titanium boride, zirconium boride, hafnium boride, molybdenum boride, tungsten boride, tantalum boride, and other metal borides, etc.
It is another object of the present invention to provide a technique for fabricating silicide ceramics with CVD method at low temperature (preferably under 250-degC.) and high growth rate ( greater than 1 micron/hour, preferably about 10 micron/hour) by using silanes and metal-releasing agents as precursors. The silicides are preferably titanium silicide, zirconium silicide, hafnium silicide, molybdenum silicide, tungsten silicide, tantalum silicide, and other metal silicides, etc.
It is another object of the present invention to provide a technique for fabricating complex/composite ceramics with CVD method at low temperature (preferably under 250-degC.) and high growth rate ( greater than 1 micron/hour, preferably about 10 micron/hour) by using metal-releasing agents as precursors. Examples of the complex/composite ceramics include titanium aluminum nitrides, titanium carbonitride, mixture of nitrides and carbides, layer by layer structures (e.g. one layer carbide over another layer of nitride), mixture of at least two ceramic materials including nitrides, carbides, borides, oxides, suicides, sulfides, and carbon, hydrogenated carbon materials, etc.
It is another object of the present invention to provide a technique for fabricating wear resistant and tribological coatings on fuel injector, hydraulic valve components, machine tools (e.g. cutting inserts, drills, mills, punches, dies, saws, molds, pins, knifes, cutters, etc.) at temperatures lower than 500-degC., preferably lower than 250-degC. with good bonding strength or adhesion. The coatings may include the above nitrides, carbides, oxides, borides, carbon, hydrogenated carbon materials, fluorides and sulfides based lubricants and their composites.
It is another object of the present invention to provide a technique for fabricating low friction-coefficient coatings ( less than 0.15, and preferably smaller than 0.1 or even lower, without lubrication against steel) on fuel injector components, hydraulic components and machine tools. The coating materials may be selected from the group of the above nitrides, carbides, borides, oxides, intermetallics, lubricants such as MoS2, WS2, fluorides, carbon, polymers.
It is another object of the present invention to provide a technique for fabricating hard coatings on fuel injector components, hydraulic components, machine tools, etc. The hard coating (equivalent to  greater than 70, preferably  greater than 80 on Rockwell xe2x80x9cCxe2x80x9d scale) may include the above ceramics, carbon, hydrogenated carbon materials, and so on.
It is another object of the present invention to provide a technique for fabricating polymer-containing materials on fuel injector components, hydraulic components and machine tools. The polymeric materials are preferably polytetrafluoroethylene (PTFE, DuPont Teflon S), polyphenylene, polyethelene, polyurethane, polycarbonate, ABS, polyimides, polyamides, polystyrene, poly-perfluoroalkoxy (PFA), poly-ethylene tetrafluoroethelene (ETFE), Fluorinated poly-ethylene propylene (FEP), other fluoropolymers, and the like.
It is another object of the present invention to provide a technique for fabricating dry lubricant-containing films for the fuel injector components, hydraulic components and machine tools. The lubricants may include metal-sulfides, (e.g. WS2 and MoS2), carbon, hydrogenated carbon, BN, or the like. The lubricants may also be composites of the above materials with metals or ceramics.
It is another object of the present invention to provide a technique for fabricating composite coatings of hard ceramic material and above lubricant materials with low friction coefficients on fuel injector components, hydraulic components and machine tools. The lubricant materials are preferably selected from the group of metal sulfides (e.g. tungsten disulfide, molybdenum disulfide), carbon (e.g. graphite, glassy carbon, amorphous carbon, etc.), hydrogenated carbon, boron nitride, polymeric materials including polytetrafluoroethylene (PTFE, DuPont Teflon S), polystyrene, polyurethane, polycarbonate, ABS, polyimides, polyamides, polyphenylene, polyethelene, polystyrene, Teflon, polyimide, polyamide, poly-perfluoroalkoxy (PFA), poly-ethylene tetrafluoroethelene (ETFE), Fluorinated ethylene propylene (FEP), fluoropolymers and so on.
It is another object of the present invention to reduce material cost by fabricating fuel injector components, hydraulic valve components, machine tools and other wear parts with lower grade, less expensive materials. A suitable coating is applied on their surfaces to improve performance.
It is another object of the present invention to eliminate part or entire heat treating procedures. For example, one may coat components directly after fabrication and finishing. In the case of fuel injector components, after they are machined to size, shape and finish, coating, e.g. titanium nitride, may be directly applied without extensive heat treating, at least part of it.
It is another object of the present invention to provide a technique for applying wear-resistant coating on fuel injector components, hydraulic components and machine tools with thermal spray techniques, either in vacuum or in atmosphere. The material may be a composite of ceramic or metallic material and lubricants.
It is another object of the present invention to provide a one-step surface modification by combining the surface heat treatment process with CVD coating process. For example, titanium nitride coating may be applied with nitriding or carbonitriding equipment and process upon introducing metal-containing material. Carbon, and hydrogenated carbon coating may also be applied with carburizing or nitrocarburizing equipment and processes. Silicon carbide may be fabricated with carburizing equipment and process upon adding silane. Metal carbides may be fabricated with carburizing equipment and process upon adding metal-containing material.
It is another object of the present invention to provide a technique to improve bonding between the coating and the substrate by combining surface heat treating processes such as nitriding, carburizing and carbonitriding with coating processes. For example, freshly nitrided surface has a better bonding with nitride coatings. Freshly carburized surface has a better bonding with carbon-containing coatings. Freshly siliconised surface has a better bonding with silicon-containing coatings. Freshly boronised surface has a better bonding with boron-containing coatings.
It is another object of the present invention to provide better metal-releasing agents as precursors for CVD ceramic coatings.
It is another object of the present invention to provide a technique for fabricating nanoscale coatings with better wear-resistant properties.
It is another object of the present invention to provide a technique and precursor systems to fabricate metal-containing coatings at low temperature, preferably lower than 250-degC.
It is another object of the present invention to provide a technique to improve coating growth rate and to increase adhesion between the coating and the substrates. For example, electromagnetic radiation may be used to radiate substrate surface during the process so as to promote ionization, activation, growth rate, bonding. The radiation may be ultraviolet or infrared rays.
According to the invention, the said deposition is carried out at low temperature so that the base material would not be thermally affected or mechanically weakened. Moreover, the lower temperature required would significantly reduce preheating, cooling time and therefore total cycle time.
According to the invention, metal-releasing agents are used as precursors. The precursors, together with reactive gas(es) and plasma assisting gas(es) are introduced into an enclosure in which substrates (e.g. fuel injector or hydraulic components or machine tools) are placed. A plasma is initiated by applying a power source to the gas system. The gases are then dissociated and ionized into highly energetic species which react with substrate surface to form desired coatings.
According to an aspect of the invention, the power source may be chosen from the group of microwave power, radio-frequency, electric power including direct and alternating current, high-frequency or other wavelengths of electromagnetic radiation, electron beam, ion beam, or even laser radiation.
According to an aspect of the invention, the substrates (e.g. fuel injector or hydraulic components or machine tools) are placed in between two electrodes (e.g. two flat mesh shaped, or simply two metal plates) inside an enclosure. The precursors, reactive gas(es) and plasma assisting gas(es) are introduced into the enclosure. A high-amperage electric current (either direct current or alternating current) is applied between the two electrodes to cause gas dissociation and ionization to form highly energetic species which then react with substrate surface to form coating.
According to an aspect of the invention, the substrates may be used as one of the two electrodes.
According to an aspect of the invention, two or even more power sources may be used simultaneously.
According to an aspect of the invention, the metal-releasing agent is a titanium-releasing agent, the reactive gases are nitrogen, hydrogen and/or ammonia, the coating is a titanium nitride or titanium carbonitride or composite (titanium nitride and titanium carbide) coating.
According to an aspect of the invention, the metal-releasing agent is Ti(NR2)4, where R is an organic group such as methyl, ethyl groups, etc.
According to an aspect of the invention, the equipment for gas nitriding, carburizing, or carbonitriding may be used for coating processes by introducing metal-releasing agent.
According to an aspect of the invention, the substrates may be electrically biased to a positive or negative potential to increase ionic or electron bombardment so that adhesion and growth rate may be promoted.
According to an aspect of the invention, a magnetic field may be applied to enhance the deposition process. Particularly at pressures lower than about 10 Torr, electron cyclotron resonance (ECR) may be initiated. The ECR process can be used to promote adhesion, growth rate and uniformity of the deposition.
According to an aspect of the invention, the fuel injector components, hydraulic components, machine tools and other wear parts may be made of metal, alloy, ceramic or composite materials (e.g. steel, tool steel, stainless steel, high speed steel, tungsten carbide-cobalt based ceramics, aluminum oxide, silicon nitride, silicon oxide, tungsten carbide, etc.).
According to the invention, there is provided a low temperature, TiN coating process which features:
combined heat treating and coating processes: nitridingxe2x80x94nitride coatings (TiN, (Ti, Al)N, TiCN, (Ti, Al)CN), etc. carborisingxe2x80x94DLC.
Adhesion promotion treatment: surface activation forms a unsaturated layer (e.g. nitrogen) which forms a bridge between coating and substrate.
Nanocrystaline renders very smooth surface (see FigRoughness).
According to a feature of the invention, a resulting TiN coating exhibits: wear resistant, hardness 2800 Vickers or equivalent to Rc 84, 4 times harder than HSS; low friction co-efficient; stable up to 650-degC. in air; inertness to most chemicals; golden color. Extends tool life, increases speed.
Why low temperatures are used in the Inventive Process
Typical prior art coating processes use high temperatures. PVD works at  greater than 500-degC., CVD works at 1000-degC. These high temperatures cause several major problems. (1) softens/anneals heat treated steel; (2) causes distortion; (3) changes dimensions over time.
Cost effectiveness of Inventive Process
Currently, there are only two major processes available, high T PVD and high T CVD. PVD occupies most of the market. Both are very expensive, e.g. coating a xc2xcxe2x80x3 diameter xc3x972.5xe2x80x3 rod cost about $2. In average, post-treatment of these components cost 3 times as much as fabrication cost; TiN costs almost half of the components. It is necessary to bring coating cost down.
Uniqueness of Inventive Process
The following characteristics make the present invention distinctive from the prior arts: low temperature ( less than 350-degc); high growth rate ( greater than 6micron/hr.); plasma enhanced CVD; Ti- and N-releasing agents; and unique surface activation.
Advantageous Characteristics of the Inventive Coating(s)
Advantageous properties of the inventive coatings include: lower friction than the prior art coatings; much stronger coating-substrate bonding; drastically lower interface stress; improved uniformity; less expensive core material; shorter cycle time (no loading, evacuation, preheating, cleaning, shorter cooling cycle; high deposition rate); better reproducibility; composite coating capability; lower cost; low investment.
The process of the present invention has a unique surface activation step. Before deposition, nitrogen plasma is used to activate the surface. Basically, nitrogen-containing gas is introduced into the plasma and is dissociated and ionized to form nitrogen ions which then bombard surface, react and diff-use into the subsurface to form a thin nitride layer. This process is very similar to traditional ion nitriding process except that there is no need to penetrate that deep as they do. Lot of nitriding people need millimeter (mm) deep. The present invention benefits from merely nitriding (activating) the surface. This surface activation process generates a freshly activated surface covered by highly energetic, chemically unsaturated nitrogen atoms with dangling bonds, as depicted here.
This surface activation step results in many benefits. Because this step creates massive, energetic nucleation sites which trap the titanium ions/atoms as soon as they meet them, and therefore significantly promotes deposition rate.
Adhesion: the activated surface forms stronger chemical bonds in the coating-substrate interface.
Smoothness: large nucleation density always produces smoother coating.
More importantly, Nanocrystalline can be formed with large nucleation density. The nanocrystalline always lead to better performance because it is harder, smoother, and so on.
Ti- and N-releasing agent precursor
A unique, efficient Ti- and N-releasing agent system is used as a precursor. For example, Ti(NR2)4 where R=organic groups, e.g. ethyl, propyl, etc. The uniqueness of these molecules include (1) Tixe2x80x94N bonds are already formed; (2) Organic groups can be easily removed; (3) Ti/N ratio are better controlled; and (4) the dissociation temperature is so low that it forms TiN coating at very low temperature (250-degC.).
Applications for the present invention may include: wear parts (including actuators, gages, bearing races, valves, pistons, shafts, etc.); cutting tools (WC, high speed steel); knives; pins; cold-forming dies; surgical instruments/tools; punches; cutting and die inserts; injection mold parts; hydraulic components such as valves; sliding components; camshaft, ceramic metal forming tooling, drills, threading tools, extrusion dies, punches, medical wear parts, surgical tools and wear parts, gears, shocks, struts, bearings, pistons, cylinders.
Present invention provides a low temperature, high growth rate plasma enhanced chemical vapor deposition (CVD) process for TiN and related coatings on fuel injector components. The deposition process is carried out at low temperatures ( less than 350-degC.) to prevent the bulk substrate properties from being adversely affected. The injector components are treated with a modified xe2x80x9cplasma nitridingxe2x80x9d process before deposition. Bonding strength (adhesion) between the coating and the injector components can be improved by combining the plasma nitriding processes with the titanium nitride coating processes. TiN-solid lubricant composite coatings are also proposed for fuel injector components. Titanium- and nitrogen-releasing agents are used to improve deposition efficiency. Electric bias potential may also be employed to further enhance adhesion, growth rate and film quality. Other critical components may be treated in the same way using the present invention.
The benefits of the present inventive techniques may be summarized as follows:
1. LOW TEMPERATURE:
The present invention uses titanium- and nitrogen-releasing agents, such as Ti(NR2)4, where R stands for organic groups. The low bonding energy between N-R makes it easily broken, rendering low temperature deposition feasible.
2. HIGH GROWTH RATE: Generally, the surface activation process generates a great number of nucleation sites, which promote initial growth and eliminates the incubation period observed in most deposition processes. In addition, the unique precursors used supplies not only titanium and nitrogen sources, but also chemical bonding of titanium-nitrogen required for TiN formation, leading high growth rate.
3. LOWER FRICTION: The best way to reduce friction is to produce a smooth surface. The present inventive process has very high nucleation densityxe2x80x94a prerequisite for smooth coating. In addition, this process creates nanocrystalline films which generally furnish smooth surfaces and low friction. Moreover, the present inventive processes may produce TiN-solid lubricant based composites, which could significantly reduce friction.
4. STRONGER COATING-SUBSTRATE BONDING: Most PVD and CVD coatings have only physical bonding (van der Waals forces) between coating and substrate. In contrast, the present inventive process has an unique activation stepxe2x80x94nitriding. This treatment fabricates a freshly nitrided surface covered by active, valence-unsaturated nitrogen atoms with dangling bonds. It is these unsaturated nitrogen atoms that react with titanium to form titanium nitride films. Since bonding between these active nitrogen atoms and the subsurface is already formed, TiN coating subsequently deposited is therefore chemically bonded to the subsurface. The chemical bonds formed are ten times stronger than physical bonds, leading much stronger coating-substrate interface.
5. LOW INTERFACE STRESS: Differences in thermal expansion characteristics between coating and substrate always bring about stress at interface after the coating deposited at high temperature is cooled down. In extreme cases, the coating may peel off from the substrate. Low temperature used reduces interface stress caused by changing thermal condition.
6. IMPROVED UNIFORMITY: The freshly nitrided surface offers extremely high nucleation density; and additionally the present inventive process uses CVD technique which is omni-directional, the combination leads to very uniform coating.
7. ALTERNATIVE CORE MATERIAL: Wear-resistance is a surface-specific property, therefore it is the surface (rather than core substrate) that plays critical roles in wear related applications, presumably suitable coating has been applied on properly treated substrate with adequate thickness and adhesion. In other words, it is coating that determines how well components work in a large extent. Therefore, lower grade, less expensive materials may be used to replace more costly high speed tool steel materials. Moreover, heat treating processes may be eliminated, at least in part. For example, fuel injector components may be directly nitriding treated and coated with titanium nitride or composites, immediately after mechanical fabrication and finishing. Another possibility is to eliminate some of the heat treating procedures, e.g. quench hardening and/or tempering steps, and directly nitride and coat the parts after mechanical fabrication and finishing.
8. REPRODUCIBILITY: The present inventive process is insensitive to fluctuations of gas flow rates, pressure and temperature, making it more reproducible, easier in quality control.
9. BETTER PERFORMANCE: It is well known that extended exposure at high temperature may significantly affect properties of steel materials and also changes their dimensions, at least in certain extent. The low temperature used in the coating technology of the present invention eliminates problems associated with high temperature processes. In addition, the potential elimination of heat treating procedures preserves core toughness of the substrate material.
10. SHORTER CYCLE TIME: Since the deposition procedure is combined with the nitriding step, loading, evacuation, substrate preheating and cleaning procedures are eliminated. The low temperature used also shortens cooling cycle. Moreover, the high deposition rate cuts deposition time. The total cycle time is therefore significantly reduced.
11. COMPOSITE COATING CAPABILITY: By introducing aluminum precursors, (Ti, Al)N coating may be produced. TiN-solid lubricant films may also be fabricated using this process.
12. LOWER COST: All above advantages will transform into better performance and reduced cost.
13. LOW INVESTMENT: The present inventive technology combines plasma nitriding and titanium nitride coating processes. Most plasma nitriding may be readily modified to carried out the inventive processes. No additional equipment investment is necessary.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.